The invention relates to the manufacture of polymeric products, and more particularly to filters for polymer melts and solutions.
The invention may be most advantageously used for cleaning polymer melts obtained from extruders.
For the manufacture of various products of polymeric materials using extruders, such as substratum for photographic materials, insulating films for electric insulation purposes and capacitors, transparent sheets and yarn, clean resin is required. During preparation of polymer and its subsequent pelletizing as well as during packaging and shipment thereof to processing plants, it is impossible to avoid contamination with solid inclusions of the material which should therefore, be filtered. Filters are specially added to some of the polymers with a purpose to improve their properties and quality of finished products. Most of the fillers are finely ground substances, such as titanium dioxide added to polyethylenetherephthalate.
Despite appropriate classification of the filler powder, particles of a size exceeding admissible limit enter the polymer.
Films and yarn made of polymeric materials filtered during the manufacture are characterized by the lower thickness limit of 1 to 30.mu.. Filtered particle size should not exceed the thickness of finished product and in majority of applications their size should be even smaller than that of the product thickness.
Coarse inclusions in the films and yarn result in breakage during continuous manufacture of products, increased quantity of discarded products and higher manufacturing cost.
In addition, films and yarn with coarse inclusions exhibit lower technical parameters as regards electric strength, transparence and other properties.
Filtering of polymer melts and solutions substantially differs from filtering of other contaminated liquids. Processing of melts features high viscosity (500 - 50000 Pu), relatively high filtering temperature (up to 400.degree. C), low concentration of solids, low thermal stability of majority of polymers in molten state. Throughput capacity of extruder plants equipped with filters reaches from 500 to 1000 kg/h, and the fineness of cleaning should be up to 1 - 30.mu., as mentioned above.
Combination of properties of processed material and requirements imposed on finished products calls for application of high pressure during filtering (up to 200-300 kp/cm.sup.2) It is to be noted that duration of continuous operation is 15-30 days, the period required to ensure stability of thermal performance of processing plants, reduce inoperative time of equipment and lower polymer losses associated with starting periods during which stable performance parameters are achieved. However, even at the processing temperature, continuous thermal exposure of the melt for 5-10 hours results in its destruction and impaired properties of manufactured products.
Therefore, there should be no dead zones in filters for polymer melts, and filter construction should provide for directional flow of melt.
Widely known filters and filtering plants for separation of suspensions and purification of oils are unsuitable for filtering polymer melts because of different pressure and temperature parameters involved and dead zone formation. Special filters are employed for polymer melts. Continuous filters are most promising among them.
Filters for polymer melts are divided into three fundamentally different groups:
filters having replaceable filter elements; PA1 filters with intermittent washing of filter elements; PA1 filters with continuous washing of filter elements.
It is common knowledge that the filters of the first group having replaceable filter elements comprise a casing with heaters, an inlet opening and an outlet opening, a movable partition in the form of a cylinder or plate, two replaceable filter elements located therein which are made in the form of discs, and a pneumatic or hydraulic drive system for displacement of the partition. The partition is arranged at right angle to the melt flow. With one of the filter elements being clogged, the partition is displaced and the other filter element is put in operation. The used filter element is removed to be replaced by a new one.
In such filters, small filtering surface results in cyclic changes in the melt pressure upstream the filter, i.e., from minimum value upon installation of a new filter element up to maximum value when the filter element is regarded as completely clogged. Such pressure changes result in increased difference in thickness of manufactured products. Filtering surface in such filters is within the limits of the extruder screw diameter, hence frequent replacements of filter elements. In addition, known filters are difficult to operate because of the necessity to replace filter elements at the polymer melting point (above 200.degree. C); it is likewise difficult to avoid melt leakages along the travelling area of the movable partition wherethrough polymer may enter heaters to cause their failure and contamination of manufacturing environment.
Filters of the third group with continuous washing of filter elements are based on the principle of rotation of the filtering surface relative to the inlet chamber and washing device.
A known filter possesses a casing formed with an inlet opening and an outlet opening, and accommodating two rotatable filtering discs, the axis of each disc being at right angle to the flow path. The discs are rotated by an electric motor. Each filtering disc adjoins to a fixed perforated plate secured to the casing. The casing is provided with a valve communicating with a washing liquid draining line. A melt fed through the inlet opening is passed to a part of the surface of the rotary filtering discs. A major part of the filtrate leaves through the filter outlet opening, and a part of the melt passes through the contaminated surface of the rotary filtering discs. The flow rate of the melt for washing the filter elements is controlled by means of the valve.
The use of the known filter for fine cleaning to be carried out with high throughput capacity of the polymer melt, such as polyethyleneterephthalate, is impossible due to the need in large filtering surface of the filtering discs.
It is known that with a reduction of pore size while retaining high throughput capacity, the pressure of melt upstream the filter increases, and larger filtering surface is required to lower the pressure. The provision of large rotary filtering surface is, however, hampered by great technical difficulties associated with sealing of the joint between the rotary filtering surface and fixed plate, and results also in rapid wear of the filtering surface, thus adversely affecting the quality of the filtrate.
The use of the valve for regular washing of the filter results in the formation of dead zones in the filter, hence in thermal destruction of polymer melt, thereby impairing the quality of the manufactured products.
In addition, the use of the valve results in cyclic pressure increases, hence in a greater difference in thickness of manufactured products.
The problem of filtering highly viscous polymer melts and solutions at high throughput capacity and the fineness of purification is most successfully solved by application of the filters of the second type with regular washing of the filter elements.
One of the main problems in filtering at high throughput capacity during continuous operation (up to 15-30 days) is the provision of a cartridge filter element having a large filtering surface. A developed filtering surface is obtained by providing undulations axially of or at right angle to the axis of the cartridge. Though both systems are acceptable and find a wide application in a number of filter types, most promising and perfect is the cartridge with undulation at right angle to the cartridge axis. Such a cartridge is normally composed of lenticular filter elements.
Lenticular filter elements have filtering surfaces on either side.
A screen plate having a through central hole coaxial with the through hole of the filter element is accommodated in the inner space of the filter element. This hole receives a sleeve having through radial passages communicating the inner space of the filter element with the inner space of the sleeve which is connected to the outlet opening of the filter.
The filtering surface defined by cermet discs is welded along the periphery. A plurality of such lenticular elements are supported by a hollow mandrel to form a cartridge having a developed filtering surface. They are axially fixed by means of clamping flanges.
In order to prevent contaminated solution from leaking through the filter elements, there is provided a seal located on the sleeve of the filter element.
Such cartridges having developed surface are successfully used in filters for a polymer melt, operating at high throughput capacity for sufficiently long time.
Where troubles arise in the manufacturing process in the production of films or yarn, the filter elements should be washed.
In addition, during the starting period of operation, after replacement of the filter and upon a new starting cycle, washing of the filter elements is also required.
Contaminations of filtering surfaces result from overheating and destruction of polymer due to the long-term residence of melt in the filter at a high temperature.
The use of such filters in extruders for manufacturing lines in the production of films and yarn is, however, complicated due to the fact that there is no provision for washing the filter with reversed melt flow directly in the filter. This is due to the fact that the weld zone fails upon feeding melt to the inner space of the filter elements.
Development of filters with a large filtering surface with a possibility of washing directly in the filter becomes especially important for fine and superfine filtering where the quantity of entrapped particles increases, and clogging of pores is substantially accelerated, while the filtering time is reduced. The construction of one of the known filters almost solves the problem of washing filtering baffles. This known filter comprises a casing formed with an inlet opening and an outlet opening and accommodating a hollow mandrel. The mandrel has through radial passages. Filter elements located on the mandrel are axially fixed.
The filter element has two filtering baffles. A metal ring is located between the filtering baffles on the periphery thereof. The ring is welded to the baffles so as to define a space accommodating spacer plates. The spacer plates have perforations and a central hole coaxial with the through holes of the filtering baffles forming the filter element.
The spacer plates adjoin the filtering baffles. A sleeve having through radial passages provides communication between the inner space of the filter element with the inner space of the sleeve which is connected to the outlet opening of the filter and extends through the central holes of the baffles and plates.
The filtering baffles are welded to the sleeves, and the sleeves are welded to the mandrel in such a manner that their radial passages are in register with those of the mandrel.
A plurality of such elements fixed to the hollow mandrel form a cartridge having developed filtering surface.
Auxiliary perforated plates adjoining the filter elements are mounted between the filter elements. Cartridges having such filter elements may be used for filtering polymer melts and solutions at high throughput capacity and during continuous operation.
Contaminated polymer is admitted under pressure to the filter casing through the inlet opening. After the polymer fills the inner space of the filter, the melt or solution is squeezed through the filtering baffles of the filter elements fixed to the hollow mandrel. Filtrate flowing along the spacer plates leaves the inner space of the filtering elements through the radial passages of the sleeves to pass through the hollow mandrel and leave through the outlet opening of the filter.
This known construction of the filter cannot provide for reliable contact of the spacer plates with the filtering baffles. This is explained by the fact that there are inevitable manufacturing thickness tolerances in making filtering baffles and spacer plates. The thickness not only differs from plate to plate, but each spacer plate or baffle has different thickness at various points of its surface.
Dimensional deviations in thickness result in the absence of intimate contact between the filter elements and spacer plates after assembly so that the filtering baffles are axially displaceable to result in weaker interconnection of the filter elements along the periphery. This results in failure of filter elements during washing of the cartridge due to breakages in the weld zone.
In addition, welding of the ring along the periphery of the filtering baffles also results in weakening of the weld zone due to appearance of internal stresses thus also lowering the strength of the filter elements. Therefore, pressure difference across the filtering baffles cannot be increased, filter recovery is incomplete, and service life of the filter is short.
During filtering, the filtering baffles are clogged in time to such an extent that their further operation becomes inexpedient. Due to the solid construction of the filter, the entire cartridge is to be replaced, while only filtering baffles are "worn" so that the filter is economically inadvantageous.
Attempts to provide an economically advantageous filter resulted in the development of a filter having a demountable cartridge construction. The filter comprises a casing formed with an inlet opening and an outlet opening, accommodating a filtering assembly. The filtering assembly comprises two flanges, filter elements located between the flanges, each element consisting of two filtering baffles having through central holes. A spacer plate having a central hole coaxial with the holes of the filtering baffles is located between the baffles, the spacer plate also serving for sealing the filtering baffles along the periphery. The filtering element also comprises a sleeve having through radial passages, which is located on the periphery of the central hole of the filtering baffle. The surface of the spacer plate adjacent to the filtering baffle has a plurality of passages communicating with the through central hole of the baffle. Spacer sleeves are mounted between the filter elements on the periphery thereof, which have through radial passages, and a sealing ring is provided on the periphery of the through hole of the filtering baffles. The filter elements are axially kept in position by means of flanges and their radial displacement is limited by shoulders of the spacer sleeves.
The filter elements, spacer sleeves and sealing rings are axially pressed together between flanges which are tightened by means of a stud and a nut. One end of the stud is screwed in an auxiliary flange adjacent the cartridge flange. The auxiliary flange has through passages communicating with the radial passages of the sleeves of the filter element and with the outlet opening of the filter.
This filtering assembly may be used for filtering polymer melts and solutions at high throughput capacity during continuous operation. Contaminated liquid is admitted under pressure to the filter casing through the inlet opening. After the contaminated liquid has filled up the inner space of the casing, it is fed through the radial passages of the spacer sleeves to the surface of the filter elements. Filtrate separated at the filtering baffles flows through the passages of the spacer plates and through radial passages of the filter element and thence flows via the openings of the auxiliary flange of the cartridge to the outlet opening of the filter.
Such a filter for filtering polymer melts and solutions has a short service life because it does not enable washing of the filtering assembly directly in the filter for the filtering baffles fail due to their deformation upon feeding viscous liquid to the inner space of the filter elements.
The known construction of the filter cannot provide for reliable filtering of polymer melts and solutions with fine and superfine purification due to the lack of intimate contact, that is due to the formation of an uncontrollable gap between each spacer plate and filtering baffle after the assembly of the filtering group because of the difference in thickness of these components. This factor results, after all, in penetration of contaminants to filtrate.
In addition, this construction results in appearance of dead zones, e.g. between the shoulders of the spacer sleeves and in other places.